Changes between Version 104 and Version 105 of Documentation/TrunkFunctionality4

Timestamp:

2021-09-06T20:29:06+02:00 (3 years ago)

Author:

luyssaert

Comment:

--

Documentation/TrunkFunctionality4

@@ -157 +157 @@
 Describes r6783. The flag '''ok_bare_soil_new''' controls how the bare soil is perceived and calculated. If set to FALSE the total bare soil is still calculated veget_max_1 + sum(veget_max_i - veget_i) with i from 2 to the number of PFTs. When a deciduous PFT sheds its leaves, the gaps in the forest will thus contribute to the bare soil fraction in the grid. Although this approach was introduced a long time ago to get acceptable evaporation estimates from forest, the approach also resulted in using the albedo of PFT1 deserts as the background albedo of forest gaps. In ORCHIDEE v2.1 the background albedo has been reparameterized and this issue may have been largely resolved now if '''alb_bg_modis''' = y ([https://forge.ipsl.jussieu.fr/orchidee/wiki/Documentation/TrunkFunctionality4#Albedobackgroundr6614 more details]). From many points of view a dynamic bare soil fraction is strange, e.g., bare soil has its own water column so when moving the forest gaps from to forest to PFT1 the soil water content, soil carbon content, litter layer, etc all changes temporary. If '''ok_bare_soil_new''' is set to TRUE, canopy gaps no longer contribute to the bare soil. The new albedo scheme (see Albedo and Background albedo) considers a specific background albedo for each PFT and calculates the albedo of the PFT including the canopy gaps but the calculation of bare soil evaporation underneath a canopy would be problematic. For that reason ok_bare_soil_new is recommended only to be used with the multi-layer energy budget (when run with more than 1 layer). The multi-layer energy budget accounts for within canopy turbulence and can therefore deal with evaporation from beneath a canopy. At present the settings for ok_bare_soil_new are included in the energy_control flag ([https://forge.ipsl.jussieu.fr/orchidee/wiki/Documentation/TrunkFunctionality4#Singlevsmultilayerenergybudgetr6614 more details]). The default settings combine the new albedo scheme with the single layer energy budget (enerbil) and '''ok_bare_soil_new''' = n.
 
+=== Bare soil evaporation ===
+The calculation of vbeat4 in r7278 was (numerically) confirmed to be identical to the calculation of vbeta4 in ORC2 for a pixel with 100% bare soil coverage. At sun set the bare soil evaporation starts to deviate between both models because differences in the albedo calculation. When prescribing the ORC2 albedo in r7278 the exact same bare soil evapotranspiration is obtained for the whole 10 years of the test run. When running global simulations there is a 40% higher bare soil evaporation in r7278 compared to ORC2. Approximative factorial simulations with fixed albedo and fixed veget_max suggest that the difference in veget for vegetated PFTs contributes most to the difference in bare soil evapotranspiration. Given that veget is calculated through Pgap, the nature of Pgap seems to be the cause.
+
+ORC2 calculates veget with Lambert Beer and vbeta4 was most likely tuned to obtain reasonable bare soil evaporation (is it reasonable to start with? CAn it be tuned?). ORC4 uses Pgap which is from an ecological point of view superior to Lambert Beer because it accounts for clumping and could account for stand structure. When combing Pgap with ORC2 vbeta4, bare soil evapotranspiration becomes too high. In the long term, Pgap should be kept and the whole definition of "bare soil"should be revised. This may happen as part of the multi-layer energy budget calculations.   
+  
 
 === Bark beetles ===